Microelectromechanical systems (MEMS) oscillators are commonly used to generate clock signals for electronic devices. A typical MEMS oscillator comprises an amplifying circuit, a resonator that furnishes a native resonance frequency in feedback around the amplifying circuit, and a tuning circuit that tunes the resonant frequency to produce a clock signal at a target frequency. The resonator can be, for instance, a piezoelectric based acoustic resonator, and the tuning and amplifying circuits can be, for instance, a complementary metal oxide semiconductor (CMOS) device.
MEMS resonators are valued because they exhibit much lower loss than resonators that could be realized with conventional processes. The MEMS resonators can be combined with standard integrated circuits in various manners and this combination allows for a more robust design space, increasing the range of available tradeoffs between power consumption, phase noise, and finished product size.
The tuning range of a MEMS oscillator is typically limited to a narrow band of frequencies, and this band is generally constrained by physical properties of the oscillator. Such properties can include, for instance, the native resonance frequency of the resonator, explicit capacitances associated with capacitors or varactors used to tune the oscillator, and parasitic capacitances associated with co-located features of the oscillator. Because all these additional capacitances only lower the frequency of oscillation, the native resonance frequency of the MEMS resonator must be higher than the final desired frequency of the oscillator. Additionally, these adjustable capacitances exhibit increased loss along with increased capacitance.
The tuning range of the MEMS oscillator may also be limited by operating conditions of the oscillator, such as the supplied bias power level. For example, as less power is supplied to the oscillator, the amplitude of its output becomes smaller, which places limits on the amount of attenuation it can withstand from lossy elements such as capacitors and varactors. These limits on the lossy elements can further limit the tuning range of the oscillator.
The tuning range of the MEMS oscillator can also vary based on random factors such as variances in manufacturing processes and changes in temperature during operation. As an example, the resonator may be designed to have a native resonance frequency close to a target frequency of the oscillator. However, due to random variations in manufacturing processes, the actual resonance frequency of the resonator can vary across a range of values. For instance, it may vary according to a Gaussian distribution. If the actual resonance frequency is at a lower end of the range, a relatively small amount of capacitance may be required to tune the oscillator, which can allow the oscillator to operate at a relatively low power level. On the other hand, if the actual resonance frequency is at an upper end of the range, a larger amount of capacitance may be required to tune the oscillator, which may require the oscillator to operate at a higher power level.
Because many electronic devices operate on a strict power budget, it is generally beneficial to minimize power consumption of basic components such as MEMS oscillators. However, due to the above limitations, designers of conventional MEMS oscillators may be required to sacrifice tuning range to achieve lower power consumption.